Circulating tumor cells (or CTCs) are rare cells present in the blood of metastatic cancer patients. Quantitative detection of CTCs is important for early detection of cancer as well as monitoring of the disease progression and response to therapy. CTC count correlates with overall tumor burden and can often serve as more reliable indicators of metastatic disease than molecular disease markers. For example, the level of prostate specific antigen (PSA) can often rise due to benign prostate hyperplasia (common in people over 60) and hence may not necessarily indicate cancer.
The presence of a significant number of CTCs can be a reliable indicator of the presence of cancer. Also, possible recurrence after surgery can be detected much earlier by CTCs than by most molecular markers. Another advantage is that CTCs can be further interrogated after detection; sequencing of the genome and transcriptome could reveal the mutations that had led to cancer as well as the expression levels of the genes in question. The cells can also be cultured, grown and tested with different combinations of chemotherapeutic agents for drug discovery and personalized medicine.
Detecting CTCs however is a challenging task because of their scarcity in blood samples, as few as single cell in multiple milliliter (mL) blood samples. The current favored approach to detecting whole cells in clinical and laboratory settings is flow cytometry, wherein labeled cells are detected as they flow in single file through an optical detector. This technology is used widely from vaccine analysis to monitoring of AIDS. However, the high cost and large size of flow cytometers usually limits this testing approach to central facilities shared by many users. Furthermore, since the cells have to pass through a sensing portion of the flow cytometers in a single file manner, volumetric sample throughput is relatively low and cytometers need to run for long times to analyze large samples. For so-called “rare” cells—i.e., cells that are scarce in a fluid sample, such as CTC cells—relatively large volume samples may be required to find the cells. In this instance, the current flow cytometers can be prohibitively expensive for frequent diagnostic usage.
Microfluidic cell detectors have been developed to overcome the cost and size limitations of traditional flow cytometers in certain applications. These sophisticated systems can successfully interrogate small samples, on the order of μLs (microliters), but such systems have been found to have limited capability for analyzing large samples, on the order of multiple mLs. Most microfluidic systems offer good performance in analyzing small, microliter- or nanoliter-sized sample volumes. However, because of their micrometer dimensions, microfluidic “lab-on-a-chip” detectors need many hours to process large, milliliter-sized sample volumes. Slow flow rates in microfluidic assays are usually a consequence of the microscale dimensions of the sensing channels. These dimensions are necessary to increase the probability of a rare cell (i.e., a CTC) binding on the walls of the microchannels and in some cases to increase the signal-to-noise ratio of the underlying detection mechanism. Thus, the prior microfluidic cell detectors can be generally inefficient and can require prohibitively long analysis times for analyzing the large volume samples necessary for the detection of rare targets like CTCs.
In one microfluidic detection system developed by the Toner and Haber groups of Massachusetts General Hospital, a lab-on-a-chip is populated with antibody-functionalized 100 μm diameter posts spaced 50 μm apart to create fluid flow paths. In another chip design, the posts were replaced with a herringbone structure to actively assist mixing of the cells and increase their probability to bind the functionalized walls. In these studies, flow rates used with clinical samples were on the order of 1 mL per hour, at which rate processing a typical 7.5 mL blood sample could take many hours. In order to reduce the fluidic transport times to a manageable level of minutes rather than hours, the flow rate through these prior systems would have to be increased by one or two orders of magnitude. In general the necessary modifications to prior microfluidic systems can be problematic because: 1) the fluidic resistance of the micron-sized flow channels and the associated macro-to-micro connections would be very high; 2) a high flow rate through a small cross-sectional area would result in a high “linear speed” which would create shear stresses beyond levels that could be sustained by the antibody/cell binding on the device wall and lead to detachment of the cells; and 3) too high a linear speed would detrimentally affect the capture efficiency of the target cells in the first place. Increasing the size of the channels would allow higher flow rates but this would significantly reduce the probability of the target cells' interaction with the functionalized walls.
In the case of other microfluidic devices that use electronic detection techniques, larger dimensions would reduce the device's detection sensitivity since most microdevices need some form of focusing of targets onto a small sensor area for detection. Other researchers have parallelized their microfluidic detectors (many micro-channels side by side) to overcome the throughput problem. However, the flow rates that are used can be on the order of only 10 microliters (μLs) per minute, which can lead to hours of time to process the large volume samples necessary for CTC detection.
A high-throughput yet relatively simple and robust rare cell detection system would be highly beneficial in many research and clinical settings. Therefore, a sensor apparatus is needed that can detect rare cells, such as CTCs, in whole blood in a high-throughput manner by which sample fluids at rates of milliliters per minute (as opposed to microliters per minute) are processed to capture the contained cells. Such a system would also be highly useful in detecting various other types of cells, bacteria and spores present in sample fluids or in the environment.